Electrical converters are well known in the art and are available in many configurations for a variety of applications. Generally speaking, a converter converts an electrical supply of one type to an output of a different type. Such conversion can include DC-DC, AC-AC and DC-AC electrical conversions. In some configurations a converter may have any number of DC and AC ‘parts’, for example a DC-DC converter might incorporate an AC-AC transformer converter section.
More specifically, ‘inverter’ is a term that can be used to describe a DC-AC converter. An inverter may exist in isolation or as part of a larger converter (as in the above example, which must invert the DC to AC prior to the AC-AC transformer). Therefore, ‘converter’ should be interpreted to encompass inverters themselves and converters that include inverters. For the sake of clarity, the remainder of this specification will refer only to ‘converter’ without excluding the possibility that ‘inverter’ might be a suitable alternative term in certain contexts.
There are many configurations of converters that achieve DC-AC conversion. Predominately, this is through a suitable arrangement of switches that by means of co-ordinated switching cause current to flow in alternating directions through a component. The switches can be controlled by control circuitry to achieve a desired AC output waveform. Further circuit components can be included to shape the output waveform. Subject to the particular circuit topology, the output waveform will be dependent on the switches' frequencies, duty-cycles and working interrelationship.
One example of the use of converters is in the context of inductive power transfer (IPT) systems. These systems are a well known area of established technology (for example, wireless charging of electric toothbrushes) and developing technology (for example, wireless charging of handheld devices on a ‘charging mat’). Typically, a primary side generates a time-varying magnetic field from a transmitting coil or coils. This magnetic field induces an alternating current in a suitable receiving coil that can then be used to charge a battery, or power a device or other load. In some instances, it is possible to add capacitors around the transmitter coil to create a resonant circuit. Similarly, capacitors can be added around the receiver coil(s) to create a resonant circuit. Using a resonant circuit can increase power throughput and efficiency at the corresponding resonant frequency.
Ordinarily, the transmitting coils are driven by a converter. The characteristics of the driving current (such as frequency, phase and magnitude) will be related to the particular IPT system. In some instances, it may be desirable for the driving frequency of the converter to match the resonant frequency of the resonant transmitting coil and/or the resonant receiving coil. The magnitude may be changed to correspond to the load requirements on the secondary side. In some systems, the load requirements can be communicated to the primary side by a suitable means.
All of these layers of control add complexity and cost to the design of IPT systems. Accordingly, it is desired to have a simplified method of controlling a converter.
Another problem associated with IPT systems, as outlined in U.S. 2008/0211478 A1 (Hussman et al), is that for resonant systems, the resonant frequency of the transmitter is not fixed but varies according to the load on the receiver. Changes in the load are reflected back to the transmitter through the mutual inductive coupling, which in turns affects the resonant frequency of the transmitter. Thus, if the converter is supplying an output to the transmitter coil at a frequency that is no longer equivalent to the resonant frequency of the transmitter the power throughput is diminished and the system becomes less efficient.
A further problem associated with IPT systems is that the values of resonant components such as the transmitter or receiver coil and the resonant capacitors may vary due to manufacturing tolerances, age, temperature, power transmission distance changes and the presence of nearby metal or magnetic material, among other factors. These variations affect the resonant frequency of the transmitter, which may fall out of resonance with the receiver causing power throughput to be diminished and the system to become less efficient.
One way that this variation in resonant frequency can be accommodated is by adapting the control switches to switch off and switch on when the voltage through the transmitting coil goes to zero. Thus, the switching frequency will automatically correspond to the resonant frequency of the transmitting coil. A disadvantage of such a solution is that the frequency of the transmitted magnetic field will then vary over a range dependent on the resonant frequency of the transmitting coil. This is problematic for two reasons: first, the receiver must adaptively retune to changes in the transmitted frequency or alternatively lose power; and secondly, it is undesirable to have the system operating over a range of frequencies since the available bandwidth might be too narrow.
It is an object of the invention to provide a method for controlling a converter which allows for more simplified control of the electrical characteristics of the output of the converter such that it can still respond to changes in resonant frequency, or to at least provide the public with a useful choice.